1. Field of the Invention
This invention relates to a photosensitive resin composition which is useful as a protective insulating film for semiconductor devices, an alignment film for liquid crystal devices and an insulating film for multi-layered printed circuit boards. The invention also relates to a method for making a polyimide film patterned in a desired form on various types of substrates.
2. Description of the Prior Art
Polyimides or polyimide compositions are known as a heat-resistant photosensitive material. For instance, a negative photosensitive material is well known in the art wherein polyamic acid precursors are introduced at the carboxyl groups thereof with photo-crosslinking groups through ester bonds (Japanese Laid-open Patent Application Nos. 49-115541 and 55-45746) or with ionic bonds (Japanese Laid-open Patent Application No. 54-145794). On the other hand, positive photosensitive polyimide materials have also been reported including a material wherein o-nitrobenzyl esters are introduced at the side chains of polyimides (J. Appl. Polym. Sci., 33, 1763 (1987)), or polyamic acid silyl esters to which an onium salt is added as a photosensitive material.
Positive photosensitive polyimides which can be developed with an alkaline aqueous solution may be utilized in existing photoresist developing devices and the film of the polyimide is substantially free of swelling at the time of the development.
The positive working mechanism of the photosensitive polyimides is as follows: when the polyimide is exposed to light, the portions where exposed become more soluble in an alkaline aqueous solution; and when the exposed polyimide film is developed with an alkaline aqueous solution, the exposed portions are dissolved out thereby forming a pattern. With known positive photosensitive polyimides, however, non-exposed portions of the resin are dissolved in the alkaline aqueous solution to some extent. This eventually leads to a reduction in film thickness at the non-exposed portions. Thus, the film thickness varies after the development, presenting the problem that the dimensional accuracy is not good. Moreover, the solubility of the exposed portions appreciably differs depending on the type of polyimide used as a starting material. Some types of materials may be low in sensitivity with a lowering of resolution.